Cryogenic air separation plants require a source of clean, dry air for sustained and safe operation. This means that the moisture and carbon dioxide in the plant feed air must be removed to a very low level. Such removal reduces the problem of heat exchangers becoming plugged with ice and solid carbon dioxide, an occurrence which can prematurely cause plant shutdown. Further, any hydrocarbons contained in the feed air need also to be removed.
Feed air can be cleaned of contaminants by adsorption. Thermal swing adsorption (TSA) and, more recently, pressure swing adsorption (PSA) systems have been used for contaminant removal. TSA air prepurifier systems used in air separation plants have relatively long cycle times (measured in hours) and blowdown losses between cycles are thus relatively insignificant. By contrast, PSA cycle times are relatively short (measured in minutes) and their more frequent blowdowns represent a relatively significant loss of pressurized feed air. In addition, during cycle changes there is an accompanying upset in the process. This upset requires a finite amount of time to counteract, causing further losses in efficiency.
The design of a PSA prepurifier system is generally based upon given conditions for a particular application or, more specifically, a given air separation plant under design. The design conditions are conservatively chosen so that a desired rate of product can be produced under most of the expected ambient conditions. A fixed time cycle is generally chosen to fit these conditions as closely as possible. Naturally, the actual conditions of operation will vary, day to night, day to day, and season to season.
All of these variations affect the performance and efficiency of operation of the PSA prepurifier. The cycle time can be adjusted somewhat from time to time.
However, in actual practice this is frequently not done or just overlooked. Thus the design of the system will often provide for larger beds than would actually be required for a normal or average day. A certain amount of the bed will thus remain unused for many cycles. This results in higher capital cost and also in higher operating costs because of a higher frequency of cycling, with greater blowdown losses.
The use of PSA air prepurifiers for cryogenic air separation plants is well established. As indicated above, prepurifier cycle times are generally fixed and are based on assumptions regarding ambient and equipment parameters. Thus, calculations of adsorptive capacity based on total air flow and keyed on the breakthrough of carbon dioxide are used to determine cycle time for a particular application. Further, to be conservative, the calculations are based on an assumption of the hottest day temperature. Such calculations result in a fixed cycle time during which only a portion of the adsorptive capacity of the bed is used. Once the fixed cycle time was established, it thereafter remained constant.
An example of a timed-cycle PSA system may be found in U.S. Pat. No. 5,042,994, Smolarek, J., "Improved Control of PSA Operations". The Smolarek process uses a variable volume nitrogen product storage vessel that is monitored to determine variations in demand. The cycle is adjusted during periods of reduced demand to maintain the desired product purity and pressure with power reduction and energy savings being achieved under turndown conditions.
Another example of a time-controlled PSA system is found in U.S. Pat. No. 4,810,265, Lagree et al., "Pressure Swing Adsorption Process for Gas Separation". This is an improved PSA process which enables the more readily adsorbable component of a feed gas to be economically recovered, e.g. nitrogen from air. The more readily adsorbable component is used as a copurge at an upper adsorption pressure and less readily adsorbable component is used for countercurrent purge at a subatmospheric desorption pressure and for initial repressurization in a countercurrent backfilling step. The sequence of operations is time-controlled.
U.S. Pat. No. 5,258,056, Shirley, A. I., "PSA System with Product Turndown and Purity Control" describes a control system which maintains a desired purity while adjusting for changes in product demand in a PSA system. A sensing device detects a change in product gas demand, the signal from which is compared to a standard which then varies the feed rate of the gas entering the system.
U.S. Pat. No. 4,725,293, Gunderson, J., "Automatic Control for Pressure Swing Adsorption System" describes a PSA system which uses a constant cycle time and a control system to modify air input flow to insure that the produced nitrogen contains only a preselected range of impurity (consisting essentially of oxygen) and to keep the output flow of nitrogen relatively high.
U.S. Pat. No. 4,693,730, Miller,G. Q., "Pressure Swing Adsorption Product Purity Control Method and Apparatus" describes a method for automatically controlling product purity in a PSA process, thus preventing impurity breakthrough as feedstock changes. The process senses a characteristic of the effluent from concurrent depressurization and takes corrective action. Among suitable corrective actions are: 1) adjusting adsorption step time to control impurity loading of each bed; 2) adjusting the concurrent depressurization termination pressure to control impurity breakthrough at the product end of each bed and/or 3) adjusting the amount of purging gas received by each bed to control the extent of regeneration.
EPA 250235, Armond, J. W. et al., "Improvements in and Relating to PSA Oxygen Generating System" describes a system which senses oxygen concentration at the outlet of adsorption beds, while producing oxygen. An oxygen sensor output, located close to the outlet of each bed, varies with time as a function of the exhaustion of a bed. The control system monitors the oxygen concentration with time to determine the time to switch beds.
Accordingly, it is an object of the invention to provide an improved method for control of the cycle time of a PSA air prepurifier.
It is a further object of the invention to provide a method for control of the cycle time of a PSA air prepurifier wherein substantially continuous control is exerted in dependence upon inlet air feed parameters.
It is a still further object of the invention to provide a method for control of the cycle time of a PSA air prepurifier which enables a maximal use of adsorbent beds during cycle times.